Method for modifying pore size distribution zones in fiber cement composites and articles of manufacture of the same

ABSTRACT

A cementitious product and method of modifying the properties of a low or medium density FRC product by providing a predetermined pore size distribution. The pore size distribution is obtained such that in critical zones of the distribution, the pore volume is substantially equivalent to or less than the pore volume in a respective critical zone of a conventional high density FRC product. The resultant material provides improved properties over conventional medium density FRC products, in particular improved freeze/thaw durability and/or improved workability.

This application is divisional application under 35 U.S.C. §121 to U.S.application Ser. No. 10/530,770 filed Oct. 6, 2005, now U.S. Pat. No.7,993,570 a National Phase application filed under 35 U.S.C. §371 ofInternational Application No. PCT/AU2003/001315, filed Oct. 7, 2003,which claims the benefit of U.S. Application No. 60/417,076, filed Oct.7, 2002 and Australian Application No. 2003901529, filed Mar. 31, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to fibre reinforced cement products andmethods for modifying the properties of those products for particularpurposes.

2. Description of the Related Art

Any discussion of the prior art throughout the specification should inno way be considered as an admission that such prior art is widely knownor forms part of common general knowledge in the field.

Generally, fibre reinforced composites may be divided into three groupsbased on their density.

High density reinforced composites have a density range above 1.6 toabout 1.9 g/cm³. These composites may be formed in a conventionalfashion involving matting of fibres, catchment of slurried fines anddewatering, eg the Hatschek process followed by compression of up to 30MPa pressure to the desired thickness.

Such high density FRC materials have high structural strength, highstiffness and a smooth finish. One particularly desired advantage ofhigh density products is their ability to resist moisture ingressthereby retain as-manufactured physical or chemical properties inservice.

Unfortunately, however, many high density FRC products do not have goodhandlabilty, nailability and can be difficult to score and snap. Thereis also a high capital and maintenance cost involved in the productiontechnique.

Medium density FRC products with a density from about 1.2 to 1.6 g/cm³overcome some of the difficulties mentioned above. Even though they arenormally formed in a conventional fashion, eg Hatschek process, they canbe produced for relatively low cost compared with high density FRCproducts, have improved workability, ie handleability, score-snap,nailability and provide adequate structural strength and stiffness formost applications. Further, they generally have acceptable in serviceperformance.

Conventional medium density FRC products, however, may not generallyhave the same level of resistance to moisture ingress and ability tomaintain in-service performance as high density products. Further, theymay not provide the flat smooth surface produced on high densityproducts without additional coating and/or sanding.

Low density fibre reinforced composites with a density of around 0.8 to1.1 g/cm³ are also formed in a conventional fashion, e.g. Hatschek andnormally incorporate a density modifier.

These low density products have excellent workability, i.e.handleability, score and snap and nailability due to their low density.They provide acceptable in-service performance and have adequatestiffness.

Such low density products, however, generally have lower structuralstrength and stiffness. Ability to maintain physical and chemicalproperties in service is generally lower and once again, surfaceflatness could be improved. Due to the specialised formulation of mostlow density fibre reinforced composites, they are produced at a relativehigh cost.

Accordingly, it will be appreciated by persons skilled in the art thatit would be useful to be able to modify the properties of a medium orlow density product such that they retain their advantageous properties,eg workability relatively low cost etc, but improve other properties, egability to maintain as-manufactured properties, resistance to moistureingress, structural strength and stiffness and surface flatness.

Performance in extreme climactic conditions is a particularly difficultarea. For example, in many geographical locations, the FRC product maybe subject to many freeze/thaw cycles during its life. Some conventionalmedium density FRC material produced may suffer from delamination,softening or chipping when exposed to freeze/thaw cycles.

Loss of as-manufactured physical or chemical properties can also betriggered by internal factors such as imperfections relating to thematerial heterogeneity, eg air pockets, segregation of constituents. Inthe case of fibre reinforced composite materials, imperfections such aspoor bond at the matrix-fibre interface and fibre clumping may rendersuch materials more susceptible to loss of durability.

It is an object of the present invention to overcome or ameliorate atleast one of the disadvantages of the prior art, or to provide a usefulalternative.

SUMMARY OF THE INVENTION

In a first aspect, a method of modifying the properties of a low ormedium density FRC product comprises providing the low or medium densityFRC product with a predetermined pore size distribution such that inparticular critical zones of said distribution, the pore volume issubstantially equivalent to or less than the pore volume in a respectivecritical zone of a conventional high density FRC product.

In a preferred embodiment, the predetermined pore size distribution isobtained by chemical modification, physical modification, or acombination of chemical and physical modification.

In another aspect, the pore size distribution is obtained by including apredetermined quantity of pore modifying components into thecementitious formulation. In one embodiment, the pore modifyingcomponents include lignocellulosic fibres treated with a water repellentagent and microfine siliceous material such as silica fume.

Alternatively, or in addition to such chemical alteration of the poresize distribution, the low or medium density FRC product may besubjected to a light press to provide the required predetermined poresize distribution. The pressure applied to the low or medium density FRCproduct is sufficient to provide the desired predetermined pore sizedistribution and provide a density of no greater than about 1.6 gms percm³. Preferably, the density of the resultant FRC product is betweenabout 1.1 and 1.55 grams per cm³.

The present applicants have found that there are a range of propertiesof the low or medium density FRC product which may be altered bycontrolling the pore size distribution of the resultant product. Theyhave also discovered that it is not necessary to control the entire poresize distribution but rather said distribution in critical zones, eg inthe region of 1 to 10 microns mean pore diameter size and in the regionof 10 to 100 microns mean pore diameter size. These regions, sometimesreferred to as the fibre pore and air pore regions, are critical to someof the resultant properties of the low or medium density FRC product.The applicants have found that it is possible to provide a pore volumein these critical regions which is no greater than around 150% of thepore volume in a corresponding pore size range of a conventional highdensity FRC product while still maintaining a low or medium density, iea density below about 1.6 g/cm³. The resultant product has improvedproperties over conventional materials and in particular improveddurability in a freeze/thaw environment.

In a preferred embodiment, the predetermined pore sized distribution isobtained to provide improved freeze/thaw durability of the FRC product.

In another embodiment, the predetermined pore size distribution isobtained to provide reduced propensity to carbonation or differentialcarbonation of the FRC product.

In a further aspect, the present invention provides a method ofproviding a low or medium density FRC product, comprising the steps of:

i) providing a formulation for a low or medium density product;

ii) forming a green article from said formulation;

iii) curing said green article to form said product;

wherein prior to step iii)

a) pore size modifying agent(s) are added to the formulation and/or

b) the green shaped article is subjected to a light press

such that the pore volume of the product is reduced as compared with aconventional product emanating from steps i) to iii) while maintaining adensity of no greater than about 1.6 g/cm³.

In still a further embodiment, step (i) mentioned above can be providedinto stages A formulation for a medium density product may be preparedand a density modifier added to reduce the density of the formulationbelow 1.1 to 1.55 grams per cm³. Light pressing then via step (b) bringsthe product back towards the medium density range of about 1.1 to 1.55grams per cm³.

In still a further aspect, the present invention provides a method ofmodifying the properties of a low or medium density FRC articlecomprising adjusting the pore size distribution of the article such thatover a particular pore size range, the total pore volume is no greaterthan about 150% of the pore volume in a corresponding pore size range ofa conventional high density FRC product.

The present invention also provides a cementitious product comprising abody constructed from fibre reinforced cement and having a density of nomore than about 1.6 grams/cm³ wherein the product has a predeterminedpore size distribution such that in particular critical zones of saiddistribution, the pore volume is substantially equivalent to or lessthan the pore volume in a respective critical zone of a conventionalhigh density FRC product.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example only, withreference to the accompany drawings in which:

FIG. 1 is a graph of the freeze/thaw performance of high-density(pressed) and medium density (unpressed) conventional FRC composites;

FIG. 2 is a graph of the pore size distribution of the conventional FRCcomposites of FIG. 1;

FIG. 3 is a graphical representation of the pore volumes in respectiveranges for two FRC composites and an FRC composite produced according toone embodiment of the present invention;

FIG. 4 is a graph of the freeze/thaw performance of high-density(pressed) and medium density (unpressed) conventional FRC composites andan FRC composite produced according to a first embodiment of the presentinvention;

FIG. 5 is a graph of the pore size distribution of the FRC compositeshown in FIG. 4;

FIG. 6 is a graph of the freeze/thaw performance of high-density(pressed) and medium density (unpressed) conventional FRC composites andan FRC composite according to a second embodiment of the presentinvention, and

FIG. 7 is a graph of the pore size distribution of the FRC composite ofFIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Fibre reinforced cement typically comprises

i) A binder such as Portland cement, which is prepared, for instance, byadding a cure modifier such as calcium sulfate (gypsum) to a clinkerprepared by firing a raw material consisting of iron oxide, quartzite,clay, and lime (CaO) at a high temperature and then pulverizing themixture. Examples of Portland cement include early strength Portlandcement, ultra-high early strength Portland cement, moderate-heatPortland cement, sulfate-resisting Portland cement, and white Portlandcement. Additionally, examples of the binder other than Portland cementinclude blast furnace cement, silica cement, fly ash cement, and aluminacement. Range: about 10% to 60%, preferably about 20% to 50%, mostpreferably about 30% to 40% by total weight.

ii) Pozzolanic materials: Man-made pozzolanic materials (both amorphousand crystalline) including silica fume, microsilica, metakaolin, groundgranulated blast furnace slag, and fly ash. Other naturally derivedmaterials which, when finely divided, have been referred to aspozzolanic include pumice, perlite, diatomaceous earth, tuff, trass, etc

iii) Siliceous material (preferably crystalline), the siliceous materialmay be present in an amount of from about 10-80 wt %, preferably about30-70 wt %, preferably about 40-65 wt %. Preferably the siliceousmaterial is ground sand (also known as silica) or fine quartz althoughamorphous silica is also suitable. Preferably the siliceous material hasan average particle size of about 1-50 microns, more preferably about20-30 microns.

iv) Reinforcing fibres: Suitable fibrous materials capable of producinga fibre reinforced product include cellulose such as softwood andhardwood cellulose fibres, non wood cellulose fibres, asbestos, mineralwool, steel fibre, synthetic polymers such as polyamides, polyesters,polypropylene, polyacrylonitrile, polyacrylamide, viscose, nylon, PVC,PVA, rayon, glass, ceramic or carbon. (vegetable, ceramic or polymeric),ranging between about 0.1% to 15% by total weight of composite solids,preferably about 5% to 12%, most preferably about 7% to 9%. Preferably,the reinforcing fibres comprise of cellulose fibres which areunrefined/unfibrillated or refined/fibrillated cellulose pulps fromvarious sources, including but not limited to bleached, unbleached,semi-bleached cellulose pulp. The cellulose pulps can be made ofsoftwood, hardwood, agricultural raw materials, recycled waste paper orany other forms of lignocellulosic materials. Cellulose fibres can bemade by various pulping methods. In the pulping process wood or otherlignocellulosic raw materials such as kenaf, straw, and bamboo, etc.,are reduced to a fibrous mass by the means of rupturing the bonds withinthe structures of lignocellulosic materials. This task can beaccomplished chemically, mechanically, thermally, biologically, or bycombinations of these treatments. When cellulose fibres are used, theyare preferably refined to a degree of freeness of between about 0 and800 Canadian Standard Freeness (CSF), more preferably about 200-500 CSF.

v) Other additives/fillers, FRC composites can contain about 0-40 wt %of other additives such as fillers such as mineral oxides, hydroxidesand clays, metal oxides and hydroxides, fire retardants such asmagnesite, thickeners, colorants, pigments, water sealing agents, waterreducing agents, setting rate modifiers, hardeners, filtering aids,plasticisers, dispersants, foaming agents or flocculating agents,water-proofing agents, density modifiers or other processing aids.

The fibre cement composites disclosed in preferred embodiments of thepresent invention may be formed from the water borne slurry by any of anumber of conventional processes such as the Hatschek sheet process.

After forming, the green article may be pre-cured for a short timepreferably up to about 80 hours at up to about 60° C. maximumtemperature and high humidity, then it may be cured according to one ormore of the following curing regimes:

Air curing: at up to about 60° C. maximum temperature and high humidity.

Steam curing: preferably in a steam environment at up to about 90° C.maximum temperature and atmospheric pressure for about 3 to 30 hours,most preferably for less than about 24 hours.

Autoclaving, preferably in a steam pressurised vessel at about 120 to200° C. for about 3 to 30 hours, most preferably for less than about 24hours.

The length of time and temperature chosen for curing is dependent on theformulation, the manufacturing process and form of the article.

With conventional high density FRC composites after forming and prior tocuring, the product undergoes a pressing step where high pressure, ie upto about 30 MPa is applied to the formed article to give the desiredthickness and density. The intention of this pressing is to reduceporosity, minimise water ingress, enhance interlaminar bonds andincrease the resistance to delamination.

FIG. 1 is a graph of a freeze/thaw cycle test comparing a conventionalpressed fibre cement composite to an unpressed fibre cement composite.It can be seen that the unpressed composite exhibited significantlyfaster inter-laminar bond degradation (falling below 0.70 Mpa in 10cycles) compared to a conventional high density pressed fibre compositewhich survives 80 cycles before it loses inter-laminar bond to the samedegree.

The pore size distribution of the conventional pressed and unpressedfibre cement composites are shown in FIG. 2.

This pore size distribution can be broken down into five major groupingsas follows.

i) Air Pores (100-10 microns). These relate to macro pores caused bypoor packing, fibre clumping, dewatering etc. Sometimes they are simplyreferred to as cracks or interlaminar pores.

ii) Fibre Pores (10-1 microns). These relate to pores inherent in thelignocellulosic fibres specifically due to their tubular structure andstraw-like shape.

iii) Meso Pores (1-0.1 microns).

iv) Capillary Pores (0.1-0.01 microns). These relate to poresoriginating upon depletion of free water in the matrix.

v) Gel Pores (0.01-0.001 microns). These pores relate to the cement orbinder micropores and are quite small in size and difficult to modify.

The Applicants have hypothesised that the properties of the fibre cementcomposite, in particular durability and workability in extreme climacticconditions may be linked to the pore size distribution in the resultantarticle and most particularly to the pore size distribution in the100-10 micron size range (air pores) and 10-1 micron size range (fibrepores). Examinations of prior art fibre cement composites which exhibitgood durability and workability in extreme climatic conditions appearsto confirm relatively low pore volume in the critical air pore (100-10microns) and fibre pore (10-1 microns) regions. Previous techniques ofhigh pressure pressing, using synthetic polymeric fibres or highadditions of micro silica to reduce pore volumes in these regions, has,as discussed above, proved expensive or has reduced workability.

Workability is generally defined as the ease to transport, handle, cutby score and snap, fix, eg nail and install the FC composite. Generally,workability is inversely proportional to density, ie composites with alower density generally improve workability compared to higher densityones.

However, durability is generally directly proportional to density, iehigh density composites exhibit better durability compared to lowerdensity ones. Durability is generally regarded as the ability of thefibre cement composite to resist failure and continue performance in thepresence of flaws (delamination or cracks) or other forms of damage ordegradation for a specified period of time under specified environmentalconditions. Such degradation systems include cyclic freeze/thaw orheat/rain, premature aging, microbial or chemical attack.

Accordingly, it will be clear to a person skilled in the art that thedesired attributes of workability and durability cannot be met by thesimple prior art mechanism of adjusting density. Rather, the Applicant'saim is to provide an FC composite with good durability and workabilityby modifying the pore size distribution at least in critical zones ofthe distribution, while maintaining a density lower than about 1.6g/cm³.

EXAMPLE 1 Lightly Pressed Medium Density Composite

In a first embodiment, a medium density composite product is produced intwo stages. The first involves applying a density modifier to aconventional medium density FRC formulation to bring density down to thelow density range, ie about 0.8 to 1.1 g/cm³ thereby achieving improvedstress relaxation behaviour and workability.

In this embodiment, the density modifying agent is microspheres butother density modifiers may be used. Microspheres can be natural,synthetic or a by-product. The material can be crystalline but is moretypically amorphous or glass. One preferred type of microspheres arehollow ceramic microspheres commonly known as cenospheres. Cenospheresare a coal ash by-product that is typically separated from fly ash by afloatation process where the spheres float to the surface of water fromclarifiers, ponds or lakes. The microspheres are available, for example,under the names Extendospheres, Recyclospheres and Zeeospheres, and areavailable from suppliers such as PQ Corporation of Chattanooga, Tenn.;Zeelan Industries Inc./3M of St. Paul, Minn.; Sphere Services, Inc. ofOak Ridge, Tenn.; The microspheres have typical particle sizes rangingfrom about 12 to 300 microns, with median particle sizes ranging about80 to 120 microns. These sizes can, of course, vary between samples. Thepreferred microspheres typically contain about 62%-65% silica (SiO₂),about 23%-26% alumina (Al₂O₃) and about 3.0% to 4.0% iron oxides(Fe₂O₃). Range: about 1% to 30%, preferably about 2% to 20%, mostpreferably about 5% to 15% by total weight). Additional examples offormulating fibre cement composites using microspheres may be found inU.S. application Ser. No. 09/803,456 filed 9 Mar. 2001, entitled FIBERCEMENT BUILDING MATERIALS WITH LOW DENSITY ADDITIVES, the entirety ofwhich is incorporated herein by reference.

A fibre cement composite is then produced using a waterbourne slurry ofthe density modified formulation by any conventional process. In thisexample and those discussed below, the Hatschek sheet process is used inwhich laminations of the formulation are applied to build up thethickness of the desired product. At this stage, the FC composite isstill in the low to medium density range, ie about 0.8 to about 1.2.

The resultant FC composite is then subjected to a light press to densifythe composite to a density within the medium density range, ie up toabout 1.6 g/cm³. This achieves improved delamination and waterpermeation resistance.

It should be understood, however, that this light press is notequivalent to the high pressure pressing of the prior art. According toone embodiment of the present invention, the content of densitymodifiers and extent of pressing are manipulated to provide the desiredpore size distribution while still maintaining a density of less thanabout 1.6 g/cm³. As discussed above, in the prior art, conventional highdensity FC composites apply pressures to achieve densities of over about1.6 g/cm³. This conventional technique does improve durability but maysubstantially decreases workability. The applicants have found that itis possible to improve various properties of the FC composite includingdurability and workability by starting with a low density formulationand applying a light press to form a medium density composite.

The light pressing regime according to one embodiment of the presentinvention can be divided into three parameters namely i) maximumpressure applied—between about 5 and 40 MPa, preferably about 10 to 30and most preferably about 15 to 20 MPa, ii) ramping cycle—between about10 and 40 minutes, preferably about 15 to 35 and most preferably about20 to 30 minutes, and holding cycle—about between 15 and 30 minutes,preferably about 10 to 20 and most preferably about 5 to 10 minutes.

After pressing, the green article is precured for a short time,preferably up to about 80 hours at up to about 60° C. maximumtemperature and high humidity, then autoclaved, preferably in a steampressurised vessel at about 120 to 200° C. for about three to 30 hours,preferably less than about 24 hours.

As discussed above, other methods for curing the composite may be used,eg air curing or steam curing. The length of time and temperature chosenfor curing is dependent upon the formulation, the manufacturing processand form of the article.

The resultant light pressed medium density FC composite was thenanalysed to determine its pore volume. FIG. 3 is a comparative of thepore volumes of two medium density composites produced according to thepreferred embodiments of the present invention and two conventionalproducts. The first conventional product (A) is a pressed high densityFC composite produced using the Hatschek process and generally used inroofing applications under moderate free/thaw exposure. This article ispressed using a maximum pressure of 30 MPa with a press cycle of 15minutes ramping and 15 minutes holding.

The second conventional product (B) is an unpressed medium densitycomposite, once again produced via the Hatschek process and suitable foruse in roofing applications in mild climatic conditions. Sample (C) is alight pressed medium density FRC composite produced according to theabove mentioned process. It can be seen from FIG. 3 that in the air porerange (70-10 microns) and fibre pore range (10-1 microns) the porevolume of (C) the light pressed medium density FC composite iscomparable to the pore volume in the equivalent critical zone(s) of thehigh density product. The conventional medium density impressed product,in the other hand, has a much higher pore volume in the air pore andfibre ranges.

Sample (D) is a medium density composite produced using a modified blendand will be discussed below under example 2 in more detail.

Test 1—Freeze/Thaw Durability (Example 1)

The durability of the two conventional composites versus the lightpressed medium density composite was compared.

The first and second products are the conventional medium-density FCimpressed composite and high density FC composite outlined above.

The light pressed product is produced according to the process ofExample 1 above, namely, a light pressed medium density FC compositeonce again produced using the Hatschek process. The product is lightpressed using a maximum pressure of 15 MPa with a press cycle of 30minutes ramping and 5 minutes holding.

The formulations for each are shown in Table 1.

TABLE 1 Formulations for unpressed, high density and light pressedcomposites (% by total weight) Fire Ceramic Silica Retardant CeramicMicro- Formulation Cement Flour Pulp Filler Filler spheresMedium-density 35.0 53.0 8.0 4.0 — — Unpressed FC - (Conventional) Highdensity 39.6 48.4 8.0 4.0 — — FC - 30 MPa (Conventional) Light Pressed35.0 45.0 8.0 2.0 2.0 8.0 medium density FC - 15 MPa - (Example 1)

The freeze/thaw performance was tested as follows: FC Samples (44 mm×44mm squares) representing the three compositions were placed on one edge,half submerged in water in a plastic container, then exposed tofreeze/thaw (F/T) cycling in an environmental chamber. The F/T regimeconsisted of 4 cycles per day; each cycle involved freezing and thawingof the sample over 6 hours including 1 hour freezing at −20° C. and 1hour thawing at 20° C. The extent of degradation in the samples due tofreeze/thaw exposure was determined by tensile testing in thez-direction to determine the Inter-Laminar Bond (ILB) strength (after 0,10, 20, 40 and 80 cycles), which is a measure of the extent ofdelamination. A 0.7 MPa ILB limit was chosen as a lower limit measure ofdegradation due to freeze/thaw exposure.

The freeze/thaw performance and pore size distributions of all 3products are shown in FIGS. 4 and 5 respectively.

It can be seen from FIG. 4 that once again, the ILB strength for theconventional Impressed medium density FC composite degrades below 0.7MPa at around 10 cycles. The conventional high density pressed FC asdiscussed above, lasts for approximately 80 cycles. Most surprisingly,the light pressed medium density FC composite produced according to thepreferred embodiments of the present invention continues above the 0.7MPa line even after 80 cycles. Indeed as a comparison, both theconventional pressed high density FC composite and light pressed mediumdensity composite of the preferred embodiments of the present inventionexhibit around a 700% improvement in freeze/thaw durability compared tothe standard FC composite.

Turning to FIG. 5, it can be seen that both the conventional highdensity FC composite and light pressed medium density composite of thepreferred embodiments of the present invention exhibit lower porosity inthe air pore zone (100-10 microns) and fibre pore zone (10-1 microns) ascompared with conventional medium density products.

The fact that the freeze/thaw durability exhibited in the medium densityFC product of the preferred embodiments of the present invention exceedsthat of the conventional high density pressed composite is quitesurprising and unexpected. The composite according to the preferredembodiments of the present invention exhibits lower density and hence,it is expected to perform worse under freeze/thaw conditions as comparedwith a conventional high density product. Further, both the conventionalhigh density pressed composite and light pressed medium densitycomposite of the preferred embodiments of the present inventionexhibited comparable interlaminer bond strengths under substantiallydiffering pressing regimes. This is quite surprising and contrary toconventional wisdom in this area.

Not wishing to be bound by any particular theory, it is speculated thatthe significant improvement in freeze/thaw durability exhibited in theembodiment of the invention discussed above, results from the combinedeffects of

-   -   reduced moisture ingress due to reduced porosity in at least the        critical zones of the pore size distribution. This is as a        result of the light pressing    -   enhanced stress relaxation in the lightly pressed modified        composite due perhaps to the presence of the density modifying        microspheres, which may enable it to accommodate more of the        destructive stresses associated with freezing and thawing.    -   higher fresh interlaminar bond strength and increased        delamination resistance due to pressing.        Test 2—Mechanical Properties (Example 1)

The above mentioned test shows that the FC composite produced inaccordance with the preferred embodiments of the present invention has asignificant improvement in freeze/thaw durability as compared withconventional medium density FC composites. Indeed, the durability iscomparable with conventional high density FC composites. As discussedabove, however, such conventional high density FC composites may havereduced workability, ductility, nailability, etc compared to theirmedium density counterparts.

Accordingly, a conventional high density FRC composite and a lightpressed medium density FRC composite produced according to the preferredembodiments of the present invention were evaluated for their flexuralproperties.

250 mm×250 mm squares were tested in flexure (in air-dry conditions) intwo directions. The flexure testing data is shown in Table 2 below.Unless otherwise stated all the density values disclosed herein relateto air-dry (equilibrium) condition, with an approximate moisture contentrange of 5%-10% by weight.

TABLE 2 Flexure testing data (air-dry condition) Avg. B-A Ult. DensityMoR Energy strains MoE Formulation gm/cm³ MPa KJ/m³ um/m GPaConventional 1.68 27.43 2.31 2631 12.86 High Density (within highPressed FC density range) (30 MPa) Light 1.46 23.41 12.97 7793 6.78Pressed (within Medium medium Density FC density range) (15 MPa)

Table 2 shows that the ductility, ie ultimate strain to failure, of thelight pressed medium density FC composite according to the preferredembodiments of the present invention is around 300% that thancorresponding to the conventional high density pressed FC, ie increasedfrom 2631 um/m to 7793 um/m. This is a very surprising result andindicates that a light pressed medium density composite according to thepreferred embodiments of the present invention is potentially able towithstand much higher strain before failing as compared to theconventional high density pressed FC composites.

The same could be said about the B-A (impact) energy which increased bymore than about 500% (2.31 to 12.97 Kj/m³).

While these results may not be typical of all light pressed mediumdensity composites made according to the preferred embodiments of thepresent invention, it is significant to note that preferred embodimentsof the present invention provide a light press medium density FCcomposite with improved characteristics over conventional products suchas improved durability, high ductility (easier to nail, better crackingresistance), lighter weight (improved workability and easier to handle)and improved impact resistance (hail resistant etc).

EXAMPLE 2 Modified Formulation/Blend FRC Composite

As an alternative to the light pressing technique discussed above inexample 1, in a second embodiment the properties of a low or mediumdensity FRC product are altered by modifying the typical FRC compositeformulation. This modification involves the addition of:

i) lignocellulosic fibres chemically treated with a water repellantagent,

ii) reinforcing fibers

and

ii) microfine siliceous material.

The lignocellulosic fibres suitable for use with the preferredembodiments of the present invention are chemically treated with a waterrepellent agent to impart hydrophobicity. They are provided in an amountof 0.01% to 9% by total weight of composite solids and preferably in the2 to 3% range.

Lignocellulosic fibres chemically treated with a water repellent agentto impart hydrophobicity, range: between about 0.01% to 9% by totalweight of composite solids, preferably in the about 2% to 4% range.

The lignocellulosic fibres as described in the preferred embodiments ofthe present specification is a generic term for cellulose fibres madefrom softwood or hardwood, bamboo, sugarcane, palm tree, hemp, bagasse,kenaf, wheat straws, rice straws, reed, and the like. Moreover,lignocellulosic fibre material is a generic term for the above fibreshaving such shapes as needle-like, chip-like, thin section-like,strand-like, rod-like, fibre-like, flake-like, and the like. It is to benoted that there is no particular restriction on the shape of theselignocellulosic fibres, but it is preferable that those having anaverage fibre length of about 0.50-50 mm, and an average fibre diameteror average fibre thickness of about 0.5 mm or less be used. Moreover,lignocellulosic fibres may be a mixture of two or more of the abovefibres.

Preferably, lignocellulosic fibres comprise of cellulose fibers whichare unrefined/unfibrillated or refined/fibrillated cellulose pulps fromvarious sources, including but not limited to bleached, unbleached,semi-bleached cellulose pulp. The cellulose pulps can be made ofsoftwood, hardwood, agricultural raw materials, recycled waste paper orany other forms of lignocellulosic materials. Cellulose fibers can bemade by various pulping methods. In the pulping process wood or otherlignocellulosic raw materials such as kenaf, straw, and bamboo, etc.,are reduced to a fibrous mass by the means of rupturing the bonds withinthe structures of lignocellulosic materials. This task can beaccomplished chemically, mechanically, thermally, biologically, or bycombinations of these treatments.

In one embodiment of the invention, the lignocellulosic fibres havesurfaces that are at least partially treated with a water repellentagent so as to make the surfaces hydrophobic. The water repellent agentcomprises a hydrophilic functional group and a hydrophobic functionalgroup, wherein the hydrophilic group permanently or temporarily bonds tohydroxyl groups on the fiber surface in the presence of water or anorganic solvent in a manner so as to substantially prevent the hydroxylgroups from bonding with water molecules. The hydrophobic group ispositioned on the fiber surface and repels water therefrom.

As discussed above, the reinforcing fibres for the FC compositeformulation may also be formed of cellulose. If this is the case, aportion of the cellulosic reinforcing fibres may be treated with thewater repellent agent to satisfy component (i) mentioned above. Ofcourse, if the reinforcing fibres are made from materials other thancellulose, e.g. polymer, additional treated lignocellulosic fibres arepreferably added to the formulation as component (i).

In another embodiment of the invention, each water repellent agentmolecule has a hydrophilic functional group comprising silanol (Si—OH)or polysilanol (Si—(OH)_(n), where n=2, 3 or 4) and a hydrophobicfunctional group comprising straight or branched alkyl chains oraromatic fragments. The silanol or polysilanol may be resulted from thehydrolysis of hydrolysable alkoxy fragments that attach to a siliconelement.

The water repellent agent may be applied to the fiber surfaces usingmethods including vacuum deposition, pressure spraying, dipping ortreating the fibres in aqueous or solvent solutions containing the waterrepellent chemicals.

Chemical compounds that can be used as water repellent agents include,but are not limited to:

silane derivatives of all kinds and in all formulations, alkoxylsilaneof all kinds and in various formulations, silicone emulsions of allkinds and in various formulations.

The water repellent agents can be in a dry form such as powders, or wetform such as emulsions, dispersions, latexes and solutions. Whenmultiple sizing agents are applied, some can be in dry form and othersin wet form.

The water repellent agent can comprise about 50% of the dry weight ofthe lignocellulosic fibres, most preferably, approximately 0.01 to 10%of its weight.

Further details regarding the chemical treatment of fibres usingemulsified sizing (water repellent) agents are described in copendingInternational PCT Application Number PCT/US01/29675 entitled FIBRECEMENT COMPOSITE MATERIAL USING SIZED CELLULOSE FIBRES, filed on 21 Sep.2001, and U.S. application Ser. No. 09/969,742, filed 2 Oct. 2001,entitled FIBER CEMENT COMPOSITE MATERIALS USING SIZED CELLULOSE FIBERS,the entirety of both of which are incorporated herein by reference.

The microfine silica is preferably added in the range of about 0.1 to10% and preferably about 2 to 4% of the total weight of dry solids.Microfine refers to the particles being essentially less than about 10microns and preferably less than about 5 microns. Examples includesilica fume which is an amorphous spherical silica by-product from themanufacture of pherosilica and silicone metal and refined natural microsilica. While crystalline silica can be used, amorphous silica ispreferred and the Applicant has found that best results are achievedwhen the SiO₂ content in this additive is at least about 85% by weight.

Referring back to FIG. 3 and Sample (D) which is a medium density FRCcomposite produced according to the modified formulation/blend, it canbe seen that in the air pore region (70-10 microns) the total porevolume is comparable with the high density product. The fibre porevolume (10-1 micron) is greater than the high density product, however,as will be discussed below, it is a combination of pore modification andwater repellent nature of the fibres which improves the properties ofthe modified blend FC composite.

Test 3—Freeze/Thaw Durability (Example 2)

The freeze/thaw durability for the unpressed durable blend FC compositewas tested as compared with a conventional medium density unpressedproduct and conventional pressed high density product.

The first and second composites are identical to the comparativeexamples given above in Table 1. The third composite is an unpressedmedium density modified blend according to example 2 and produced viathe Hatschek process.

The formulations of the three composites are shown in Table 3 below.

TABLE 3 Formulations for unpressed (standard), unpressed (durable), andhigh density pressed composites (% by total weight) Component 2Component 1 Untreated Water Component 3 Reinforcing Treated RepellentTotal Microfine Fire Silica Fibre Fibre Additive Fibre Silica (Silicaretardant Formulation Cement Flour (cellulose) (Cellulose) (Silane)Content Fume) (Filler) Medium - 39.6 48.4 8.0 0.0 0.0 8.0 0.0 4.0density FC - Unpressed (Conventional) High density 35.0 53.0 8.0 0.0 0.08.0 0.0 4.0 FC - 30 MPa (Conventional) Modified 30.0 60.0 2.70 2.90 0.325.6 30. 0.0 Blend Medium Density FC (Example 2)

Samples representing the three composites were tested under cyclicfreeze/thaw as described in example 1. The extent of degradation in thesamples due to freeze/thaw exposure was determined by tensile testing inthe z-direction to determine the Inter-Laminar Bond (ILB) strength(which is a measure of the extent of delamination) after 0, 10, 20, 40and 80 cycles.

The freeze/thaw performance and pore size distributions of all 3products are shown in FIGS. 6 and 7 respectively.

It can be seen that the unpressed durable blend FC composite inaccordance with the preferred embodiments of the present inventionprovide a significant improvement in freeze/thaw durability over theconventional unpressed medium density FC composite.

Indeed, both the conventional high density FC composite and unpressedmodified blend medium density composite according to the preferredembodiments of the present invention exhibit about 700% improvement infreeze/thaw durability compared to the standard FC composite. Bothachieve around 70 freeze/thaw cycles above the failure line.

Turning to FIG. 7, it can be seen that as will Example 1, theconventional high density pressed FC product and modified blend FCcomposite according to the preferred embodiments of the presentinvention exhibit significantly lower porosity in the air pore zone.(100-10 microns) and fibre pore zone (10-1 microns) as compared with theconventional medium density product.

The improvement in freeze/thaw durability exhibited by the modifiedblend FC composite according to the preferred embodiments of the presentinvention is surprising and quite unexpected.

Analysis of the interlaminar bond strength, capillary porosity and waterabsorbtivity of the modified blend FC composite according to thepreferred embodiments of the present invention as well as the twoconventional comparative FC composites was conducted. The results areshown in Table 2 below.

Conventional wisdom predicts that the freeze/thaw durability improvementarising from high density pressed FC products stems from the high freshMB (2.30 MPa) and low porosity of the densified matrix (air-drieddensity of 1.7 g/cm³). This is caused by the high pressure pressing ofthe product prior to curing. It is therefore surprising that themodified blend FC composite of the preferred embodiments of the presentinvention which does not undergo pressing and exhibits a low fresh ILB(1.2 MPa) and high overall porosity (air-dried density of 1.3 g/cm³) canmatch the freeze/thaw durability improvement of the conventional highdensity product.

In addition, since the modified blend unpressed FC composite of thepreferred embodiments of the present invention would normally be lighterin weight, lower in cost and more workable, ie easier to handle and cut,than the conventional high density FC product, while at the same timeexhibiting comparable durability, it presents an attractive alternativeto conventional materials.

Not wishing to be bound by any particular theory, the Applicantspeculates that improvement in freeze/thaw durability of the unpressedmodified blend FC composite according to the preferred embodiments ofthe present invention, arises due to the combined synergistic effect ofthe hydrophobic cellulose fibres and the blocked or segmented matrixpores due to silica fume reactivity. This combination increases theresistance to moisture ingress thereby improving freeze/thaw durability.

The above explanation is supported by the water/absorption data shown inTable 4 in which the conventional high density pressed FC composite andunpressed modified blend FC composite according to the preferredembodiments of the present invention exhibit between 20 and 30% lowerwater absorption values compared to the conventional impressed mediumdensity FC product.

TABLE 4 Porosities and ILB values of the three FC composites.Inter-Laminar 48 hour Water Bond Strength Absorption, Composition (ILB),MPa % weight High Density Pressed FC 2.30 26.03 Convention Unpressed FC1.05 32.38 Unpressed Modified Blend FC 1.20 23.04

FIG. 7 also clearly shows that a conventional high density pressed FCproduct and unpressed modified blend FC composite according to thepreferred embodiments of the present invention which both show goodfreeze/thaw durability, exhibit significantly lower pore volumes in thefibre pore zone (10-1 microns) and air pore zone (100-10 microns)compared to the unpressed medium density FC composite.

Test 4—Moisture Resistance (Example 2)

As a further analysis, moisture resistance parameters corresponding toconventional unpressed material and the modified blend unpressed FCcomposite of the preferred embodiments of the present invention wereassessed. The results are shown in Table 5 below.

TABLE 5 Wicking height results for unpressed durable and standard FCcomposites. Wicking height Water permeation rate after 48 hours, after48 hours Composition mm ML/hr/0.002 m² Conventional 207 113 Unpressed FCUnpressed Modified 43 31 Blend FC

The water permeation rate was measured on a 250 mm by 250 mm by 6 mmsample, laid flat and attached to a 100 mm high, 50 mm diameter Perspexcolumn filled with water and monitored for extent of permeated watervolume of 48 hours. Wicking height was measured on a 250 mm by 250 mm by6 mm sample laid on edge in a flat tray in an upright position andmonitored for wicking height progression over 48 hours. It can be seenthat the unpressed modified blend FC composite exhibited more than abouta 70% reduction in wicking height and water permeation rate compared tothe conventional unpressed FC composite.

Once again, these results are quite surprising in view of conventionalunderstanding. Water permeability may be reduced by pore filling,segmenting or pressing. Wicking on the other hand is much more difficultto control in medium density FC products which contain cellulose fibredue to their small diameter and tubular structure which promotes wickingalong the fibre direction by capillary action.

It is also surprising that the low silica fume addition level in themodified blend according to the preferred embodiments of the presentinvention is sufficient to impart significant moisture resistance anddurability improvement as compared with a conventional product. Incurrent fibre cement technology, typical levels of silica fumes are 5 to10%. The silica fume addition of the preferred embodiments of thepresent invention is around 2 to 4% and this level is generallyconsidered to low to modify the properties of the medium density FCcomposite.

Test 5—Workability (Example 2)

As with Example 1, the workability, handleability and nailability of theFC composite produced according to preferred embodiments of the presentinvention was tested. Samples representing the conventional unpressedmedium density FC product and the unpressed modified blend FC productaccording to preferred embodiments of the present invention weresubjected to flexure tests. Handleability was taken as the ultimatestrain value in the B direction corresponding to a 100 mm by 200 mm by 6mm sample tested in flexure in saturated conditions. A 5000 um/multimate strain value is generally considered the minimum for goodhandleability. Edge nailability was assessed by gun nailing at 13 mmfrom the edge onto a 250 mm by 250 mm by 6 mm sample and rating theextent of cracking. A numerical rating below 1 is considered very goodnailability.

The results are shown in Table 6.

TABLE 6 Handleability & nailability results for unpressed durable andstandard FC composites. Ult. strain Nailability b-direction rating (sat.condition), (13 mm from edge) Composition um/m (*) (**) ConventionalUnpressed FC 9095 0.79 Unpressed Modified Blend FC 11433 0.38 (*)minimum 5000 um/m for good handleability. (**) <I rates: very goodnailability.

It can be seen from Table 6 that the unpressed modified blend FCcomposite according to the preferred embodiments of the presentinvention exhibited very good handleability and nailability propertiesas compared with the conventional unpressed medium density product. Onceagain, these results are quite surprising since the modified blend FCcomposite according to the preferred embodiments of the presentinvention has a relatively low total fibre content, ie around 5.6% (seeTable 3) as compared to 8% in standard FC composites. Such a low fibrecontent is outside the range commonly adopted in cellulose FCproduction, ie 7 to 9% and accordingly, the FC composite is expected toexhibit a very low ultimate strain value, ie brittle failure, and poornailability.

Accordingly, it can be seen that the unpressed modified blend FCcomposite according to the preferred embodiments of the presentinvention surprisingly achieves an improvement in freeze/thaw durability(comparable with conventional high density FC composites) and at thesame time maintains or improves workability (handleability andnailability improves over conventional medium density products).

The embodiments shown above provide two alternatives for modifying theproperties of a low or medium density FC composite. In one embodiment, alow or medium density formulation undergoes a light press whilemaintaining a density lower than about 1.6 g/cm³. In the secondembodiment, a modified blend is used in the original formulation.

Both embodiments show significantly improved properties overconventional medium density FC products and in particular improvedfreeze/thaw durability while maintaining or improving workability. Theresultant products have industrial application in a wide range of areasincluding exterior or interior use, roofing applications, wet area FClining, etc.

While the present invention has been described with reference to theabove examples, it would be appreciated that other embodiments, formsor'modifications may be produced without departing from the spirit orscope of the invention as broadly described herein.

1. A method of modifying the properties of a low or medium density FRC,fiber reinforced cement, product comprising: applying a density modifierto a low or medium density FRC product having a density from about 1.2to 1.6 g/cm³, wherein the density modifier reduces the density of thelow or medium density FRC product to between about 0.8 to 1.1 g/cm³;densifying said low or medium density FRC product to a density of nogreater than about 1.6 g/cm³, wherein densifying the low or mediumdensity product comprises applying a light press comprising applyingabout 5 to 40 MPa to the cementitious matrix with a ramping cyclebetween 10 to 40 minutes and a holding cycle of between 15 to 30minutes; and curing said low or medium density FRC product, wherein thecured low or medium density FRC product has a predetermined pore sizedistribution such that in particular critical zones of saiddistribution, the pore volume is substantially equivalent to or lessthan the pore volume in a respective critical zone of a conventionalhigh density FRC product.
 2. A method as claimed in claim 1 wherein thepredetermined pore size distribution is obtained by physicalmodification.
 3. A method as claimed in claim 1 wherein the pressureapplied is sufficient to provide the predetermined pore sizedistribution but maintain a density of between about 1.1 and 1.55 g/cm³.4. A method as claimed in claim 1 wherein the critical zones of saidpore size distribution are in the region of 1 to 10 microns mean porediameter size and/or in the region of 10 to 100 microns mean porediameter size.
 5. A method as claimed in claim 1 wherein thepredetermined pore size distribution is obtained by a combination ofchemical and physical modifications.
 6. A method as claimed in claim 1 apredetermined pore size distribution is obtained to provide improvedfreeze/thaw durability to the FRC product.
 7. A method as claimed inclaim 1 wherein a predetermined pore sized distribution is obtained toprovide reduced propensity to carbonation or differential carbonation ofthe FRC product.